The present invention generally relates to a system and method for controlling critical dimension in a semiconductor manufacturing process and more particularly, to a system and method which independently adjusts focus and exposure settings in a semiconductor photolithography process, in order to control critical dimension.
Semiconductors are made in production facilities, commonly referred to as xe2x80x9cfabs.xe2x80x9d A large fab may contain hundreds of automated tools that cooperatively work to convert circular silicon xe2x80x9cwafersxe2x80x9d (each consisting of dozens, hundreds, and even potentially thousands of chips) into functioning products. One of the challenges in these fabs is to control manufacturing equipment and tools, such as photolithography machines, in a manner which optimizes the features of manufactured chips and minimizes variation and defects.
One of the important features to be optimized and controlled in a semiconductor manufacturing process is xe2x80x9ccritical dimension.xe2x80x9d Critical dimension may be defined as the width of a line (or a space between lines) patterned onto a wafer surface through photolithography. Critical dimension determines the size of the smallest geometric features that can be formed during the manufacture of a semiconductor device or circuit. The ability to improve the performance of semiconductor devices is highly dependent on the size of the devices, and therefore, on critical dimension. Furthermore, as the semiconductor industry drives toward smaller critical dimensions to produce faster devices, the allowable variance in critical dimension diminishes. Thus, successful control of critical dimension is crucial in semiconductor fabrication.
Some of the present systems and methods for controlling critical dimension utilize exposure energy, or dose, as a manipulated variable to affect changes in critical dimension. These techniques attempt to compensate for the effects of focus drifts on critical dimension by adjusting the exposure dose in the photolithography process. Particularly, these techniques often use a simple linear model in the control algorithm that relates exposure dose and critical dimension. For example, the following linear xe2x80x9cprocess modelxe2x80x9d may be employed by these prior control algorithms:
CDk=aEk+ckxe2x80x83xe2x80x83Equation (1)
where CDk is the critical dimension at run k, a is a predetermined parameter, Ek is the exposure dose at run k, and ck is an intercept term. The foregoing model is typically updated during the semiconductor manufacturing process by use of a simple bias or intercept adjustment. The model ascribes all variation in critical dimension to either changes in exposure or an unknown disturbance captured by the intercept term ck. While this model provides some degree of control over critical dimension, its effectiveness is limited, as it does not account for other parameters which have an effect on critical dimension.
For example, it is well known that, in addition to exposure, the focus of a photolithography tool or process also has a significant effect on critical dimension. The simultaneous effects of exposure and focus on critical dimension may be illustrated by use of conventional Bossung plots, an example of which is illustrated in FIG. 1. Particularly, these plots illustrate that there may exist numerous combinations of focus and exposure that result in the same critical dimension value. In fact, this property of photolithography is a key principle behind the known process of tool qualification, which utilizes a focus-exposure matrix. In order to create such a matrix, a test wafer is subjected to multiple combinations of focus and exposure and the resultant critical dimension values are recorded in a Bossung plot. A fabrication engineer may then use the resulting plot to choose the optimal focus and exposure settings for the photolithography process.
In view of these principles, some efforts have been made to provide systems and methods that control critical dimension by adjusting both focus and exposure settings. However, these systems and methods have all required the measurement of one or more attributes in addition to critical dimension (i.e., a second measurement) in order to make the necessary focus and exposure adjustments. Most existing metrology tools provide only a single measured attribute, i.e., critical dimension. As a result, prior art systems and methods, which require an additional measured attribute, cannot independently control both focus and exposure simultaneously using most existing metrology tools and traditional run-to-run control techniques (e.g., exponentially weighted moving average techniques). Instead, these prior art systems and methods require additional tools and modifications to simultaneously adjust focus and exposure, and are therefore inefficient and cost prohibitive.
It is therefore desirable to provide a system and method for controlling critical dimension in a semiconductor manufacturing process, which overcomes the drawbacks and limitations of prior systems and methods, which adjusts focus and exposure to control critical dimension, and which requires only a single measured attribute, such as critical dimension, to perform these adjustments.
The present invention provides many advantages over conventional system and methods for controlling critical dimension. By way of example and without limitation, the present invention accurately controls critical dimension in a semiconductor photolithography process by use of a strategy that adjusts both focus and exposure settings. Furthermore, the present invention does not require the use of any measurements other than critical dimension to adjust focus and exposure settings, and to control critical dimension in a semiconductor photolithography process. Therefore, the present invention can be implemented with existing metrology tools.
The present invention updates multiple process model parameters simultaneously in an adaptive control strategy, and systematically varies the process inputs (e.g., focus and exposure settings) from run-to-run in a manner that prevents the parameter estimates from becoming singular. Particularly, the present invention forces certain conditions upon the inputs, such as pseudorandom deviations, in order to obtain unique and stable parameter estimates.
According to one aspect of the present invention, a control system is provided for a semiconductor photolithography process. The control system is adapted to control critical dimension by adjusting focus and exposure settings within the photolithography process by use of a single measured attribute.
According to a second aspect of the present invention, a run-to-run control system for controlling critical dimension in a photolithography process is provided. The control system includes a photolithography tool for performing the photolithography process; a control law portion; and an observer portion. The control law portion is communicatively coupled to the photolithography tool, and is adapted to determine focus and exposure settings based upon a process model equation, and to further communicate the focus and exposure settings to the photolithography tool for controlling the photolithography process. The observer portion is communicatively coupled to the control law portion, and is adapted to determine a plurality of parameter values for the process model equation based upon at least one prior measured critical dimension value, and to communicate the plurality of parameter values to the control law portion.
According to a third aspect of the present invention, a method is provided for controlling an output attribute in a semiconductor fabrication process. The method includes the steps of: measuring the output attribute; controlling the output attribute by simultaneously adjusting a plurality of inputs to the semiconductor fabrication process, based upon the measured output attribute and a process model equation including a plurality of parameters; estimating updates for the plurality of parameters; and systematically varying the plurality of inputs in a manner that ensures stable estimates for the plurality of parameters.
According to a fourth aspect of the present invention, a method is provided for controlling critical dimension in a photolithography process. The method includes the steps of: monitoring a single measured attribute; and controlling critical dimension by selectively adjusting focus and exposure in the photolithography process based upon the single measured attribute.
According to a fifth aspect of the present invention, a method is provided for controlling critical dimension in a semiconductor photolithography process. The method includes the steps of: determining a focus setting by use of a best focus value and a variable deviation value; determining an exposure setting by use of a control law equation; running the photolithography process with the focus and exposure settings; obtaining a resulting critical dimension measurement; and utilizing the critical dimension measurement to update a plurality of parameters used within the control law equation.
These and other features and advantages of the invention will become apparent by reference to the following specification and by reference to the following drawings.